In the refrigeration industry, a manifold pressure gauge set is commonly used for monitoring the pressure of refrigerants contained within a refrigeration system. A manifold pressure gauge set consists of two mechanical pressure gauges attached to a common manifold. Typically, one of the mechanical pressure gauges is a low pressure gauge and the other is a high pressure gauge. Each gauge has a scale calibrated in a particular pressure unit, such as pounds per square inch (psi).
On a typical prior art gauge, such as a low pressure gauge 8 illustrated in FIG. 1 (shown removed from the manifold), the outermost scale of a gauge face is a pressure reading scale 18, and, within the pressure reading scale 18, are three smaller diameter temperature scales 12, 14, 16 calibrated in temperature units, such as degrees Fahrenheit. The pressure is read from the gauge 8 by viewing the placement of the gauge needle 17 over the pressure reading scale 18.
The gauge 8 conventionally has a removable clear plastic cover 21 that unscrews from the front of the gauge 8. The cover 21 protects the gauge needle 17 and the pressure sensing gauge mechanism from damage and contamination. The cover 21 can be removed to provide access to a calibration screw 15 of the gauge 8 and to allow replacement of the cover 21 in case of damage to the cover 21.
The gauge face 10 illustrated in the prior art gauge 8 has the temperature scales 12, 14, 16 and the pressure scale 18 printed directly on it and the gauge face 10 is typically permanently riveted into a gauge housing 13. The gauge face 10 is usually made of thin steel but plastic or aluminum are sometimes used. Ordinarily, the gauge face 10 cannot be removed from the gauge housing 13 without damage, nor can the needle 17 ordinariliy be removed from the gauge 8 without damaging the pressure sensing mechanism (not shown) within the gauge 8.
The gauge 8 also conventionally has a needle stop pin 11 at the bottom of the gauge face 10 which provides protection for the gauge mechanism if the gauge 8 is subjected to pressures substantially beyond its maximum rating. In the event the gauge 8 is subjected to such a pressure, the needle stop pin 11 will stop the needle 17 from exceeding 360 degrees travel to prevent damage to the gauge 8. The gauge face 10 also ordinarily has an access hole through which a calibration screw 15 protrudes. This calibration screw 15 may adjusted from time to time to keep the gauge accurate by returning the needle 17 to "zero gauge" when the gauge is at idle. The manifold gauge set is considered at idle when its ports are open to atmospheric pressure, and not connected to a pressurized refrigerant containing system. In this idle condition, the gauge can be calibrated to indicate "zero gauge" on the gauge scale.
Each temperature scale 12, 14, 16 on the gauge face 10 is calibrated for a specific identifiable refrigerant used in a refrigeration system, with readings on the temperature scale corresponding to respective readings on the pressure scale 18. In the prior art manifold gauge face 10 shown in FIG. 1, scales 12, 14 and 16 are provided respectively for three common refrigerants, identified as R-22, R-12, and R-502. These three refrigerants are chlorofluorocarbons (or CFCs) and contain chlorine in their chemical make-up.
Chlorine has been determined to deplete the ozone layer around the earth when released into the atmosphere. CFCs are being phased out of use because, when they are released into the atmosphere, either by a leaking refrigeration system or through the process of venting a refrigeration system, they evaporate quickly. Because CFCs are lighter than air, they eventually migrate above the ozone layer surrounding the earth's atmosphere. The intense rays of the sun break down these CFCs and the constituent chlorine molecules are released. These free chlorine molecules then chemically combine with ozone molecules at a high ratio, gradually depleting the ozone layer and allowing more harmful UV light radiation to reach the earth.
Hydrofluorocarbon refrigerants (or HFCs) have been developed to eventually replace CFC compounds as refrigerants in refrigeration systems. Because HFCs do not contain chlorine, they are considered safe for the atmosphere. HFCs do not deplete ozone molecules as do the CFCs. Unfortunately, these new HFC refrigerants are primarily designed for use in newly manufactured equipment. It is inconvenient and labor-intensive to substitute HFCs in existing equipment as a replacement for CFC refrigerants, since HFCs are not compatible with the compressor oil used in most existing equipment designed for CFC refrigerants. In order to introduce the newer ozone safe HFC refrigerants into most existing equipment as a replacement for a CFC refrigerant, the existing compressor oil must be flushed out of the system. This can be very difficult and time consuming. Also, in some older equipment, HFC refrigerant can cause some rubber seals to degenerate.
To replace CFC refrigerants in existing equipment, interim retrofit refrigerants have been developed utilizing hydrocholorofluorocarbons (or HCFCs). HCFC refrigerants have a much lower ozone depletion factor than CFCs, which means that, while they still contain chlorine, their use is much less detrimental to the ozone layer. HCFCs are expected to be permitted in an extended interim period, with eventual phase out in the year 2010. Thus, it is often recommended and easier to replace the older CFC refrigerant with an interim HCFC refrigerant.
Each refrigerant available has its own operating characteristics that make it useful and efficient for a specific application. Some of the more commonly used refrigerants are listed as follows, giving their chemical composition (i.e., CFC, HCFC, HFC) and specific applications for which they are suitable.
R-22 is a CFC refrigerant which is most commonly used in high temperature applications where the evaporator is above freezing (32 degrees), such as building and room air conditioning systems. The ozone depletion factor of R-22 is relatively small, so it is not scheduled to be phased out until the year 2020.
R-12 is another CFC refrigerant. R-12 is most commonly used in medium temperature applications where the evaporator is at or below freezing, such as refrigerators, coolers, and automotive air conditioning. The ozone depletion factor of R-12 is relatively large and as a result it is scheduled to be out of production by the end of 1995. The two main replacements which have been developed for R-12 are R-134a and R-401A. R-134a is an HFC with no ozone depleting factor and will be used in the manufacture of new equipment. It is difficult to use R-134a in existing equipment as a substitute for R-12 for the reasons discussed above with regard to substituting HFCs for CFCs. R-134a is not compatible with the compressor oil used in systems originally designed for R-12. R-401A is available as a interim replacement refrigerant for existing systems that are designed to use R-12. It is an HCFC and has a small ozone depletion factor and will be eventually phased out as the life of existing R-12 equipment expires.
R-502 is the third common CFC refrigerant. R-502 has a high ozone depletion factor and will be phased out of production at the end of 1995. Its main use is for low temperature refrigeration where evaporator temperatures are set at or below 0 degrees Fahrenheit, such as storage freezers, ice makers, and grocery store systems. The two main replacements for R-502 are R-404A and R-402A. R-404A is an HFC with no ozone depletion factor and will be available in new manufactured equipment. Like R-134a, it is difficult to use R-404A in R-502 systems as a retrofit for the reasons discussed above with regard to substituting HFCs for CFCs. R-404A is not compatible with the compressor oil used in R-502 systems. R-402A is an HCFC and has a small ozone depletion factor. R-402A is designed as an interim retrofit refrigerant for systems currently operating on R-502.
There is a direct, experimentally determined, relationship between pressure and temperature for each refrigerant. For example, the refrigerant identified as R-22, at a pressure of 30 psi, has a temperature of 7.degree. Fahrenheit. However, each of these older and newer refrigerants has a different pressure/temperature relationship. When a refrigerant system is monitored with a manifold gauge, the gauge face must have one pressure scale and several separate temperature scales. Each temperature scale is identified with and calibrated for a specific refrigerant. The temperature of a specific refrigerant can then be easily and quickly determined at whatever pressure the refrigeration system is at.
The pressure/temperature scales on these gauges also give important information about the operating conditions of the refrigerant within a refrigeration system. The low pressure gauge on the manifold pressure gauge set is used to monitor the suction or evaporator side of the refrigeration circuit. Low pressure scale typically reads from 30" Hg vacuum to 350 psi. The vacuum portion of the scale is necessary because the compressor in a system can pull a vacuum on the low pressure side of the system under certain operating conditions The low pressure scale of the manifold set is used during the installation and maintenance of a refrigeration system to set pressure controls for on/off cycling, to adjust metering devices in the system, check proper operation and refrigerant charge, and to determine the pressure/temperature of the evaporator. The pressure/temperature scales provide useful information for all of these situations. All refrigerants have a direct pressure/temperature relationship whether the refrigerant is in a liquid or gaseous state.
A high pressure gauge of the manifold gauge set is used to monitor the high pressure side (known as the condensing side) of the refrigeration system. The high pressure scale typically reads from 0 psi. to 500 psi. The high pressure scale is mainly used to monitor condensing pressure/temperatures for setting safety controls, fan cycling controls, check proper refrigerant charge, and to determine proper pumping capacity of the compressor.
For example, the appropriate temperature scale, read in comparison to the pressure scale on a low pressure gauge, is used to set the compressor of a medium temperature range refrigerator (36 to 40 degrees F.) to turn on and off at proper refrigerant pressures to prevent the contents of the refrigerator from freezing or from becoming too warm. The compressor must turn on when the refrigerator temperature reaches an upper set temperature (e.g., 40 degrees F.). The refrigerator temperature can be monitored using the pressure/temperature scale on a manifold gauge connected to the system. As the pressure of the system rises, the indicated reading on the gauge can be used to convert pressure to temperature directly on the gauge and arrive at a desired operational pressure setting to turn on the compressor and maintain the temperature from rising above the desired maximum temperature.
After the compressor system has been operating for a while, the temperature inside the refrigerator starts to fall. As the temperature falls, the indicator reading on the pressure/temperature scale can be used to convert temperature to pressure to determine a desired operational pressure setting to turn off the compressor and prevent the temperature from falling below the desired minimum temperature.
The temperature scale on the high pressure gauge, read in comparison to the pressure scale, is used to adjust proper condensing temperature, usually at 115 degrees Fahrenheit. Because of the wide range of pressures in a refrigeration system, and the need to monitor both high and low pressure sides of the system simultaneously, both high and low pressure gauges are often needed on one manifold.
With the introduction of new refrigerants and the concurrent use of the CFC refrigerants, reference scales for the new refrigerants are needed on these manifold gauge faces. Providing temperature scales on the manifold pressure gauge face for both older CFC refrigerants and newer HFC and HCFC refrigerants will make the existing manifold pressure gauge extremely useful when converting a refrigerant system from a CFC refrigerant to a HFC or HCFC refrigerant. A comparison of temperature and pressure relationship for both an older and a newer refrigerant can be made visually on the gauge face to compare the operating characteristics of a refrigerant system that is operating with a newer refrigerant as opposed to the same system operating with the older refrigerant.
Unfortunately, because of the physical size and limited space on the gauge face, there is insufficient room to include both older and newer temperature scales. One alternative is to determine the pressure reading of the refrigerant using a prior art manifold gauge, and then convert this pressure reading to the temperature for the new refrigerant by using appropriate pressure/temperature conversion charts for that refrigerant. This method is time consuming and inconvenient. Also, in converting a refrigeration system from one refrigerant to another, this method makes it difficult to compare operating pressures/temperatures for two different refrigerants.
These pressure/temperature charts provide conversion of saturated vapor pressure to temperature. With the introduction of HCFCs (interim refrigerants), the saturated vapor pressure figures are only useful under certain conditions because of a factor called temperature glide. Temperature glide is a factor existing in many of the new interim refrigerants, such as refrigerants R-401A, R-402A. R-401A and R-402A are each composed of a blend of chemicals that have different temperature characteristics, depending on whether they are in a gaseous or a liquid state, or are changing from one state to the other. For a pressure/temperature chart to be most useful, the chart must contain pressure/temperature conversions for the refrigerant both as a saturated gas and as a saturated liquid. To use the chart, the temperature of the refrigerant must be calculated when the refrigerant is changing from the gas state to the liquid state.
For example, when a refrigeration system is operating, the refrigerant in the evaporator (the cooling coil) enters as a liquid and evaporates to a gas while passing through the length of the evaporator, absorbing heat during the process of evaporating. To determine the temperature of the evaporator by using a manifold pressure gauge together with a pressure/temperature chart, it is first necessary to determine the pressure of the system using the gauge. Then reference is made to the pressure/temperature chart for that pressure. Since the refrigerant is changing state in the evaporator, the temperatures for both the saturated vapor and the saturated liquid are taken from the pressure/temperature chart. The average between these two temperatures must then be calculated to determine the average evaporator temperature in the refrigeration system. Using these pressure/temperature charts often requires estimating to within 1-9 degrees Fahrenheit (depending upon the specific refrigerant and its temperature glide factor) to arrive at the average temperature relationship in the refrigeration system being monitored.
Another alternative for determining the pressure/temperature relationship for a refrigerant would be to use two different manifold pressure gauge sets, one with old refrigerant scales and one with new. However, this alternative would prove costly and would still make accurate pressure/temperature comparisons of older and newer refrigerants difficult.